Taraxacum officinale (dandelion) is a common wildflower and garden 'weed' with bright yellow flowers and long, toothed green leaves. After flowering, fluffy seedheads known as 'dandelion clocks' develop. It's a fantastic wildlife plant – its flowers are 'composite', meaning they comprise lots of individual florets, each with its own store of nectar and pollen. They bloom early and late in the year when little else in flower, so are therefore an important source of food for insects. Its leaves are eaten by the caterpillars of several moths, along with other 'leaf munchers' like leaf miners. Its seeds are eaten by goldfinches and other birds.
Taraxacum officinale is useful to people, too. Used for centuries as a diuretic (The French word for dandelion is pissenlit, which means "pee the bed."), its leaves can be steeped in hot water to make tea and its flowers can be used to make dandelion wine. Its leaves work well in salads and its long taproot can be dried and used as an alternative to coffee.
In gardens, Taraxacum officinale is usually regarded as a weed, however more and more gardeners are embracing wilder gardens and letting dandelions flower around the edges of their plots.